zbd: use a helper to calculate zone index

zbd: use a helper to calculate zone index

zone_lock() function contains the debug code to verify that the zone
being locked belongs to the job's working zone range. Clean up this
code by using a previously defined helper for calculating the zone
index to check.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20240206105755.214891-4-dmitry.fomichev@wdc.com
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>